Smoke detector circuit

ABSTRACT

A combustion products detector system of the type having at least one ionization chamber, the combustion products sensing ionization chamber being open or accessible to the surrounding air. The output signal voltage from the chamber is connected through an FET to a four terminal voltage comparator switch, which four terminal switch has an easily adjustable threshold voltage.

United States Patent [191 Larsen et al.

[ SMOKE DETECTOR CIRCUIT [75] Inventors: Larry D. Larsen, Palatine;Edward F. Kobek, Chicago, both of Ill.

[73] Assignee: Honeywell Inc., Minneapolis, Minn. [22] Filed: July 2,1973 [21] App]. No.: 376,011

[52] U.S. Cl. 250/370, 250/390, 340/237 S, 307/252 G [51] Int. Cl. Golt1/24 Field of Search 250/390, 385, 370; 340/237 S; 307/252 G [56]References Cited UNITED STATES PATENTS 2/l97l Walthard 340/237 S 4/1942Evans 340/237 S [451 May21, 1974 Abe 340/237 S Walthard 340/237 SPrimary Examiner-Archie R. Borchelt Assislant ExaminerHarold A. DixonAttorney, Agent, or FirmLamont B. Koontz; Omund R. Dahle [57] ABSTRACT 9Claims, 2 Drawing Figures SMOKE DETECTOR CIRCUIT SUMMARY OF THEINVENTION A unique portion of the system is in the provision of a fourterminal voltage comparator switch means which has a continuouslyadjustable threshold switching level. One terminal of the switch whichmay be termed the reference terminal or threshold terminal is connectedto an adjustable reference voltage source, which in the specificembodiment shown is the wiper of a potentiometer in a voltage dividernetwork. The arrangement of the circuit provides for a switched outputsignal in response to smoke as well as an analogue output signal themagnitude of which is a function of smoke density.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 of the drawing is a schematicrepresentation of a preferred embodiment of the invention, and

FIG. 2 is a variation of a portion of the circuit of FIG. 1.

DESCRIPTION Referring now to FIG. 1 there is generally shown at anionization smoke detector circuit having a plurality of output terminalswhich are wired to a common or central station 11. The smoke detectorcircuit discloses a pair of series connected ionization chambersincluding a closed or nearly sealed reference chamber 12 and a smokesensing chamber 13 which has louvers to allow a free exchange of air inand out of the chamber so that the appearance of products of combustionor smoke in the surrounding air is quickly sensed. Reference chamber 12has electrodes 14 and 15 and the sensing chamber 13 has electrodes 16and 17. The electrodes 14 and 17 are connected across the supply voltageon conductors 20 and 21, and the intermediate electrodes 15 and 16 areelectrically connected together and provide an analogue signal output atconductor 22. Although the impedance 12 has been specifically describedas a reference chamber it is not a necessary part of the invention andmay be in the form of any suitable resistor or impedance means.

The conductor 22 is connected to the gate G of a field effect transistor23, which is shown as being an nchannel device of the junction-gate typeconnected in a source follower configuration. The drain electrode D ofPET 23 is directly connected to the positive supply conductor 20 and thesource electrode S is connected by a junction 24 and a resistor 25 tothe negative conductor 21. The junction 24 is also connected by aresistor 26 to the input terminal 30 of a four-terminal comparatorswitching means 31.

The four-terminal comparator switching means 31 is shown as comprising aPNP and an NPN transistor in I a positive feedback configuration. Inputterminal 30 connects to the emitter electrode of the PNP transistor. ThePNP base electrode is directly connected to the NPN transistor collectorelectrode and to a reference terminal 32. The PNP collector electrode isdirectly connected to the NPN transistor base electrode and to a thirdterminal 33. The NPN transistor emitter electrode is directly connectedto an output terminal 34 and terminal 34 is further connected through aresistor 35 to negative conductor 21. The reference terminal 32 isdirectly connected to the adjustable wiper of a setpoint potentiometer36. Potentiometer 36 is part of a resistive voltage divider connectedacross supply conductors 20 and 21 and adjustment of the wiper permits aselection of a desired reference or set-point voltage at terminal 32.The third terminal 33 is connected to the negative conductor 21 by adelay circuit comprising a capacitor 40 paralleled by a resistor 41.

' Output terminal 34 of the comparator or threshold switching circuit 31is directly connected to the input of a further transistor 42. Thecollector load circuit for transistor 42 is a light emitting diode LEDand a resistor 43. An output terminal 5 is also connected to thecollector electrode of transistor 42. Several other exterior terminalsof the ionization smoke detector circuit 10 are tied to various parts ofthe circuit just described, including terminal 1 which is connected by aresistor 44 to the electrode 17; terminal 2 connected to conductor 20;terminal 3 connected to terminal 32; terminal 6 connected to junction24' and terminal 7 connected to conductor 21.

Two or more of the exterior terminals of the smoke detector 10 areconnected to the common or central panel 11. Terminals 2 and 7 areconnected by conductors 50 and 51, respectively to a suitable source ofDC potential in panel 11. If desired a relay coil or coils 52 may be inseries with one of these conductors to sense changes in currentmagnitude as is well known in the art. There may be a normally energizedor actuated supervisory relay to detect a current reduction and anormally not actuated marginal relay to indicate a current increase bypulling in. The switching output terminal 5 may be connected through analarm relay coil 53 to the positive supply line. The exterior terminal 6may be connected to a voltmeter 54 to provide an indication of theanalogue voltage at junction 24 with respect to the negative source lead51 and in addition to indication this voltage may be used in a controlsystem. The exterior terminal 3 may be connected to a voltmeter 55 toprovide an indication of the differential voltage between the set pointvoltage of point 32 and the analogue voltage at junction 24, or themeter 55 may have one terminal connected to conductor 51 to indicate theset point voltage. A normally open Test switch in the central panel,when operated, connects a positive potential to the exterior terminal 1to cause the detector circuit 10 to go into alarm.

FIG. 2 shows a variation of a portion of the circuit of FIG. 1, namelythat the four terminal comparator switching means 31 is shown in theform of a silicon controlled switch (SCS). In this connection, the anodeis the input terminal 30, the anode gate is the reference terminal 32,the cathode gate is the third terminal 33, and the cathode the outputterminal 34.

OPERATION OF FIG. 1

In the normal standby condition in which there is little or no smoke inthe air being sensed by the sensing chamber 13, the effective impedanceof the two chambers l2 and 13 is approximately the same and thereforeabout half of the supply potential on conductor 20 appears on conductor22 to gate electrode G. The F ET 23 is connected as a source followercircuit and conducts sufficient current so that the analogue voltage atjunction 24 follows the analogue signal voltage at the gate. The standbyanalogue voltage at junction 24 is coupled through resistor 26 to inputelectrode 30 of the comparator switch circuit 31. The position of thewiper of set point potentiometer 36 adjusts the clean air operatingdifferential voltage. and the position is selected to be higher involtage than the standby voltage of terminal 30 so that the circuit 31is non-conductive; thus there is no voltage at output terminal 34 andtransistor 42 is also non-conductive. In this operating condition thecomparator switch 31 is stable, and due to the delay circuit connectedtoterminal 33 of switch 31, even the occurrence of a voltage transienttending to momentarily increase the potential of terminal 32, will notturn on switch 31. The potentiometer 36 makes it possible to adjust theclean air operating differential as desired at any time during the lifeof the device, and it may be desirable at specific intervals of timeover the years to check the differential and to adjust as necessary.

Upon the occurrence of smoke at sensor 13, the potential at the gateelectrode G increases above its clean air magnitude resulting in anincreased potential at terminal 30. The voltage magnitude is a functionof the 1 density of smoke. An analogue voltage exists at sensor 13, andthis analogue voltage may be measured at exterior terminal 6 withrespect to terminal 7, by a measuring and/or. control system which inmost elementary form is shown as voltmeter 54. As the potential atterminal 30 begins to exceed the reference voltage at terminal 32 due tosmoke at sensor 13, the switch 31 begins to turn on. A delay in theswitching occurs as the delay network comprising capacitor 40 charges.This delay prevents transient voltages from turning on the switch. Asthe switching is completed a positive voltage developed across resistor35 appears at output terminal 34 and biases on transistor 42. Thecollector current flowing in transistor 42 turns on the LED indicatorand also provides a low impedance path from output terminal tothenegative supply conductor 21. The relay 53 in the common panel 11 isenergized. Subsequent to the switching on of comparator switch 31, thesupply potential on conductors and 21 remains substantially constant andthe analogue voltage at terminal 24 continues to indicate the density ofsmoke at sensor 13.

If it'is 'desired'to-test the system from the common panel 11 and causethe detector to go into alarm, the Test switch is closed applying apositive voltage through resistor 4410 electrode 17 thereby raising thepotential at conductor 22. The switch 56 in the positive supplyconductor can be opened briefly to reset, or the exterior terminal 3 canbe briefly connected to the positive conductor to reset switch 31. Inoperation the switch of FIG. 2 may be explained in the same manner ashas been explained for FIG. 1.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:

1. In an ionization type products-of-combustion detector, an improvedcircuit for providing an analogue output signal voltage indicative ofthe sensed condition as well as providing a switched output indicativeof the sensed condition exceeding a set point, the improved circuitcomprising in combination;

first and second power input terminals to be energized from a dc.source;

impedance means and an ionization sensing chamber 6 analogue outputsignal voltage indicative of the sensed condition exists at said signalterminal;

a four-terminal semiconductor switch having as terminals an inputterminal, a reference terminal, a third terminal, and an outputterminal, said switch switching from an off to an on condition when thevoltage at the input terminal exceeds the voltage at the referenceterminal;

a source of adjustable set-point voltage connected to said referenceterminal;

means connected to said switch input terminal and to said chamber signalterminal and providing a signal to said input terminal which is afunction of said analogue voltage;

delay circuit means connected to said third terminal;

and output circuit means connecting the output terminal to said secondpower input terminal, said switch providing a switched output at saidoutput terminal.

2. The invention of claim 1. in which said fourterminal semiconductorswitch comprises two transistors, one an NPN transistor and the other aPNP transistor, the base electrode of the PNP transistor being directlyconnected to the collector electrode of the NPN transistor and also tosaid reference terminal, the base electrode of the NPN transistor beingdirectly connected to the collector electrode of the PNP transistor andalso to said third terminal, the emitter electrode of said PNP and NPNtransistors comprising said input terminal and output terminal,respectively.

3. The invention. of claim 1 in which said fourterminal semiconductorswitch is a four-terminal SCS (silicon controlled switch) having ananode, an anode gate, a cathode gate and a cathode as the inputterminal, reference terminal, third terminal and output terminal,respectively.

4. The invention of claim 1 in which said delay circuit means comprisescapacitive means.

5. The invention of claim 1 in which said delay circuit means comprisesa capacitor and resistive means in parallel connecting said thirdterminal to said second power input terminal.

6. The invention of claim 1 in which said impedance means is a referenceionization chamber in series with the ionization sensing chamber. 7

7. The invention of claim 1 in which said means connected to saidswitching means input terminal and to said chamber signal terminalcomprises a field effect transistor circuit.

v8. The invention of claim 1 and further comprising:

circuit means connecting said analogue output signal voltage to anexternal connection point where it can be adapted for use in control andindication.

9. In an ionization type smoke detector, an improved first and secondpower input terminals to be energized from a dc. source;

a reference chamber and a smoke sensing chamber serially connectedacross said terminals and having a signal terminal connectedintermediate said chambers;

impedance means connecting one of the current carinput terminal, thegate electrode thereof being connected to said signal terminal, so thatin operation an analogue output signal voltage indicative of the sensedcondition exists at a terminal connected to said one current carryingelectrode;

four-terminal semiconductor switch having as terminals an inputterminal, a reference terminal, a third terminal, and an output terminalin which said four-terminal switch comprises two transistors, one an NPNtransistor and the other a PNP transistor, the base electrode of the PNPtransistor being directly connected to the collector electrode of theNPN transistor and also to said reference terminal, the base electrodeof the NPN transistor being directly connected to the collectorelectrode of the PNP transistor and also to said third terminal, theemitter electrode of said PNP and NPN transistors comprising said inputterminal and output terminal, respectively, said switching meansswitching from an off to an on condition when the voltage at the inputterminal exceeds the voltage at the reference terminal;

a source of adjustable set-point voltage connected to said referenceterminal and to an output signal terminal for providing an analogueoutput signal voltage indicative of the set-point;

resistive means connecting the input terminal to said one currentcarrying electrode;

capacitive delay circuit means connected to said third terminal;

and output circuit means connecting the output terminal to said secondpower input terminal, said switch providing a switched output at saidoutput terminal.

1. In an ionization type products-of-combustion detector, an improvedcircuit for providing an analogue output signal voltage indicative ofthe sensed condition as well as providing a switched output indicativeof the sensed condition exceeding a set point, the improved circuitcomprising in combination; first and second power input terminals to beenergized from a d.c. source; impedance means and an ionization sensingchamber serially connected across said terminals and having a signalterminal connected intermediate said means and said chamber, so that inoperation an analogue output signal voltage indicative of the sensedcondition exists at said signal terminal; a four-terminal semiconductorswitch having as terminals an input terminal, a reference terminal, athird terminal, and an output terminal, said switch switching from anoff to an on condition when the voltage at the input terminal exceedsthe voltage at the reference terminal; a source of adjustable set-pointvoltage connected to said reference terminal; means connected to saidswitch input terminal and to said chamber signal terminal and providinga signal to said input terminal which is a function of said analoguevoltage; delay circuit means connected to said third terminal; andoutput circuit means connecting the output terminal to said second powerinput terminal, said switch providing a switched output at said outputterminal.
 2. The invention of claim 1 in which said four-terminalsemiconductor switch comprises two transisTors, one an NPN transistorand the other a PNP transistor, the base electrode of the PNP transistorbeing directly connected to the collector electrode of the NPNtransistor and also to said reference terminal, the base electrode ofthe NPN transistor being directly connected to the collector electrodeof the PNP transistor and also to said third terminal, the emitterelectrode of said PNP and NPN transistors comprising said input terminaland output terminal, respectively.
 3. The invention of claim 1 in whichsaid four-terminal semiconductor switch is a four-terminal SCS (siliconcontrolled switch) having an anode, an anode gate, a cathode gate and acathode as the input terminal, reference terminal, third terminal andoutput terminal, respectively.
 4. The invention of claim 1 in which saiddelay circuit means comprises capacitive means.
 5. The invention ofclaim 1 in which said delay circuit means comprises a capacitor andresistive means in parallel connecting said third terminal to saidsecond power input terminal.
 6. The invention of claim 1 in which saidimpedance means is a reference ionization chamber in series with theionization sensing chamber.
 7. The invention of claim 1 in which saidmeans connected to said switching means input terminal and to saidchamber signal terminal comprises a field effect transistor circuit. 8.The invention of claim 1 and further comprising: circuit meansconnecting said analogue output signal voltage to an external connectionpoint where it can be adapted for use in control and indication.
 9. Inan ionization type smoke detector, an improved circuit for providinganalogue output signal voltages indicative of a) the sensed conditionand b) the set point, as well as providing a switched output signalvoltage indicative of the sensed condition exceeding a set point, theimproved circuit comprising in combination; first and second power inputterminals to be energized from a d.c. source; a reference chamber and asmoke sensing chamber serially connected across said terminals andhaving a signal terminal connected intermediate said chambers; impedancemeans connecting one of the current carrying electrodes of a fieldeffect transistor to said second power input terminal, the other currentcarrying electrode being connected to the first power input terminal,the gate electrode thereof being connected to said signal terminal, sothat in operation an analogue output signal voltage indicative of thesensed condition exists at a terminal connected to said one currentcarrying electrode; a four-terminal semiconductor switch having asterminals an input terminal, a reference terminal, a third terminal, andan output terminal in which said four-terminal switch comprises twotransistors, one an NPN transistor and the other a PNP transistor, thebase electrode of the PNP transistor being directly connected to thecollector electrode of the NPN transistor and also to said referenceterminal, the base electrode of the NPN transistor being directlyconnected to the collector electrode of the PNP transistor and also tosaid third terminal, the emitter electrode of said PNP and NPNtransistors comprising said input terminal and output terminal,respectively, said switching means switching from an off to an oncondition when the voltage at the input terminal exceeds the voltage atthe reference terminal; a source of adjustable set-point voltageconnected to said reference terminal and to an output signal terminalfor providing an analogue output signal voltage indicative of theset-point; resistive means connecting the input terminal to said onecurrent carrying electrode; capacitive delay circuit means connected tosaid third terminal; and output circuit means connecting the outputterminal to said second power input terminal, said switch providing aswitched output at said output terminal.